Measurement While Drilling is a well established art and utilizes the drilling fluid stream pumped down the bore of a drill string as a communication link between instruments downhole and detection equipment on the surface. The earlier uses relied upon brief pulses in drilling fluid pressure caused by signal valves located downhole. A brief change in flow resistance was followed immediately by restoring the original condition. The pulse reached the surface for detection. The pulses were time distributed in a manner defined by the encoding preference of the designer for compatibility with surface gear.
Change-of-state signal generators cause a change in the flow resistance of the signal valve and that change alone is information. The next signal usually restores the original flow resistance. It may not be necessary to predetermine if the change is an increase or a decrease in flow resistance.
Both positive and negative pulse generators are in use. The positive signal valve usually resists the main stream of mud flow. The negative pulser usually by-passes some fluid to the well annulus through the drill string wall to reduce pressure at the standpipe. Both types may be used in the change-of-state manner described above.
Downhole apparatus may be installed in the drill string and is only recoverable when the string is tripped. Shuttle pulsers can be lowered and recovered through the bore of the drill string. The shuttle systems may carry the signal valve or may co-operate with an orifice situated near a landing baffle in the drill string that supports the shuttle downhole. Apparatus of this invention may be used either installed or incorporated into a shuttle system and may use either form of signal valve.
When mud pulse communication was new to the drilling industry drill bit life was in the order of fifty hours. Even fifty hours challenged the reliability of downhole instruments and pulsers of the time. When bit life was improved to approach two hundred hours the shuttle system was needed to permit change-out of downhole communication gear without tripping the drill string. The shuttle embodiment had to traverse the length of the drill string bore and radial dimension, already confining, was further reduced. Downhole electric energy sources, other than mud driven generators, were a limiting problem. The large surge current required for the pull-in phase of the conventional solenoid reduced life expectable from available batteries. The shuttle systems encountered life limits of a new form. When the power consumption problem is solved and greater endurance evolves in the rest of the system the need for the shuttle system may diminish because the shortcomings of the installed version will simultaneously be solved.
Solenoids have no peers for simplicity and reliability but the draw-in current requirement, to produce a specified force, is proportional to the draw-in distance. Shortening the draw-in distance approaches the amplitude of excursions of a vibrating drill string and solenoid forces have to be increased, diminishing the gains from reduced solenoid stroke. Acceleration forces one hundred times the force of gravity can be expected. There is a dire need to compensate for the acceleration and to eliminate the solenoid. Apparatus of my U.S. Pat. No. 5,020,609 issued Jun. 4, 1991 can eliminate the influence of acceleration on moving elements subject to the conventional draw-in forces. This invention addresses the need for mud powered machinery to supply the draw-in forces, allowing holding magnets with no flux gap to hold essential pulse timing elements in place until the associated downhole instrument delivers a signal to permit valve operating actions.
It is therefore an object of this invention to provide apparatus to bias the elements formerly moved by solenoids in the preferred direction by forces provided by mud powered moving elements within the pulser system.
It is a fur-her object of this invention to provide apparatus, demanding electric power for actuation, that serves only a holding function subject to the control of the associated downhole instrument.
It is yet another object of this invention to provide apparatus for mud pulse telemetry that will produce the first signal pressure increase in the absence of any downhole instrument control and will use the resulting hydraulic power to open the signal valve and retain it in a failed-safe open state until a downhole instrument exercises signal controls.
It is still another object of this invention to provide apparatus for downhole signal pressure change generation that is capable of sending signals to the downhole instrument concerning the onset of drilling fluid flow and the relative positions of the principal functional elements of the signal generator.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings.